Nonreciprocal ferrite device having a thin dielectric layer encircling the ferrite elment



Oct. 31, 1967 ET AL 7 3,350,664

NONRECIPROCAL FERRITE DEVICE HAVING YAYTHIN DIELECTRIC LAYER ENGIRCLING THE FERRITE ELEMENT Filed Feb. 11, 1966 Frequency A I I Th 'cknss Raf/o 2 Fig. 2 Angelo Pisfil/i Piefro Susin INVENTORS.

United States Patent M 3,350,664 NONRECIPROCAL FERRITE DEVICE HAVING A THIN DIELECTRIC LAYER ENCIRCLING THE FERRITE ELEMENT Angelo Pistilli and Pietro Susin, Milan, Italy, assignors to Societa Italiana Telecommuuicazioni Siemens S.p.A., Milan, Italy, a corporation of Italy Filed Feb. 11, 1966, Ser. No. 526,844 Claims priority, application Italy, Feb. 15, 1965, 3,005/65 7 Claims. (Cl. 333-1.1)

ABSTRACT OF THE DISCLOSURE Circulator with at least three waveguide sections and a ferrite core of polygonal profile at the junction thereof, the core being enveloped by a dielectric layer whose thickness is a minor fraction (preferably between 10% and 30%) of the largest transverse core dimension and which is integral with a pair of polygonal dielectric end plates projecting on all sides beyond the core, resulting in an improved circulation ratio as compared with uncoated ferrite cores.

Our present invention relates to a nonreciprocal transmitter of microwave energy comprising a waveguide wherein a gyrotropic body (i.e. a nonreciprocal gyromagnetic compound) is disposed in the path of the transmitted microwaves in the region of a unidirectional magnetic field transverse to the direction of propagation.

The effect of such gyrotropic body upon the transmitted microwave energy is to deflect it laterally in a direction depending upon the polarity of the applied magnetic field and upon the sense of wave propagation. Thus, with a waveguide having an inlet branch and an outlet branch adjoining each other at the proper angle, waves traveling from the inlet to the outlet will substantially retain their initial intensity whereas waves transmitted or reflected in the opposite direction will be sharply attenuated. In a conventional circulator with n coplanar branches (n 2), energy introduced at a first branch will be preferentially directed into the second branch whereas energy entering the guide at this latter branch will go to the third branch, and so on. A typical circulator of this type forms a Y- junction between three branches spaced 120 apart, with the system parameters so chosen that the applied magnetic field at the junction of these branches rotates the EH plane and therefore the direction of propagation by the same angle of 120.

The nonisotropic effect of the gyrotropic body, giving rise to this selective deviation, determines the energy ratio between waves transmitted in the desired or forward direction and waves traveling in the undesired or reverse direction. The general object of our invention is to provide means in such waveguide for improving this energy ratio.

A more particular object of this invention is to provide means in a circulator for minimizing reflections at the gyrotropic body by creating a proper impedance match between this body and the confronting waveguide branches.

A further object of our invention is to provide a highly compact gyrotropic structure satisfying the aforestated desiderata.

We have found, in accordance with the present invention, that these objects can be realized by the provision of a gyrotropic body which comprises, in addition to the usual ferrite core of substantially constant transverse profile perpendicular to the applied magnetic field, a dielectric layer which encircles the profile of the core and which 3,350,664 Patented Oct. 31, 1367 has a thickness equal to a minor fraction of the largest transverse core dimension.

If the insulating material constituting the encircling layer has a dielectric constant ranging between about 2 and 10, the thickness of this layer should have a lower limit of approximately 10% and an upper limit of approximately 30% of the largest transverse core dimension. Larger thicknesses may give rise to undesirable waveform modifications unless the dielectric constant is smaller than 2.

We have found, surprisingly enough, that variations in the thickness of this dielectric layer tend to modify the pass band of the circulator in the sense of lowering both the midfrequency and the upper and lower limiting frequencies as the thickness increases. Thus, the choice of a particular thickness results in the establishment of a desired frequency band.

Advantageously, the gyrotropic body may include a pair of insulating end plates which project beyond the core and are integral with the dielectric layers surrounding same, thereby forming a continuous nonmagnetic casing around the core.

Suitable dielectric materials include Teflon as well as quartz, porcelain, polystyrene and other ceramic and polymeric substances such as, for example, those marketed under the names of Rexolite and Ray-K.

Generally, the gyrotropic body and its core may assume either cylindrical or prismatic shapes. In the case of a circulator with 11 branches, for example, an advantageous profile is an n-sided polygon confronting with its corners the several branches, i.e. a preferably equilateral triangle in the case of a Y-type circulator.

Our invention will be described in greater detail with reference to the accompanying drawing in which:

FIG. 1 illustrates in isometric view (parts broken away) a Y-type waveguide circulator including a gyrotropic body according to the invention; and

FIG. 2 is a set of graphs illustrating the dependency of the pass band of the circulator upon the thickness of a dielectric laver forming part of the gyrotropic body.

In FIG. 1 we have shown a conventional Y-type waveguide 10 having three coplanar branches 5, 6 and 7 spaced apart. At the junction of these branches there is located a gyrotropic body 8 which includes a ferrite core 1 having a constant equilateral triangular profile in the common magnetic plane of the waveguide branches. A steady magnetic field, symbolized by an arrow H, is applied perpendicularly to this magnetic plane at the junction of the three branches to permeate the ferrite core 1. The polarity and intensity of this field H may be so chosen, for example, that wave energy entering the guide 10 at the input end of branch 5 is perferentially directed into branch 6 and that, similarly, waves entering at branches 6 and 7 leave at branches 7 and 5, respectively.

In accordance with this invention, a dielectric layer 4 of uniform thickness s, having a permeability of approximately unity, covers all three sides of the triangular ferrite core 1. Two triangular end plates 2 and 3 of similar material, larger than the combined profile of core 1 and coating 4, overlie the two remaining core surfaces and are preferably integral with the layer 4. It will be noted that the corners of the triangular profile lie on the bisectors of the magnetic-plane dimensions of the respective waveguide branches confronted thereby.

Tests have shown that, with conventional ceramic or plastic materials having dielectric constants between about 2 and 10, optimum results, in terms of circulation ratio and suppression of undesired harmonics or modes of propagation, are obtained if the thickness s of layer 4 ranges between approximately 10% and 30% of the length a of a side of the triangular core profile, this side constituting the largest dimension of the core in a plane perpendicular to the applied field H. The circulation ratio is defined as the ratio of proportion of energy transmitted in the forward direction and proportion of energy transmitted in the reverse direction and, with Tefion and other dielectric materials referred to above, may attain a value on the order of 40 db at the rnidfrequency of the band if the layer thickness lies within the range specified. 'Furthermore, we have found that variations of this layer thickness within that range result in definite shifts of both the midfrequency f and the lower and upper limiting frequencies f, f" of the pass band of the waveguide, this having been illustrated in FIG. 2.

As will be seen from FIG. 2, the transmitted wave energy lies in a range of about 7000 to 8000 megacycles, the midfrequency f decreasing progressively from approximately 7600 me. to about 7350 me. as the thickness ratio s/a varies from to 30%. At the same time the lower and upper limiting frequencies f, f" similarly decrease but with a slight broadening of the band toward the higher thickness ratios.

It may be mentioned that the graphs of FIG. 2, while representative of various dielectric materials and core dimensions, are based on tests made with Teflon layers 4 of dielectric constant 2.1 and with ferrite cores having a side a: 12 mm.

As the dielectric constant of the layer-forming material increases, the layer thickness s should be proportionately reduced, if the frequency band is to be maintained substantially unchanged.

Along the edges of the indicated frequency band, Whose width ff' is of the order of 10% of the midfrequency f the attenuation of wave energy in the forward direction is less than 1% whereas in the reverse direction only 0.5 to 0.7% of the input energy is transmitted; this corresponds to a circulation ratio of 21.5 to 23 db for the limiting frequencies and f".

It may be mentioned that a gyrotropic body 1 as shown in FIG. 1, stripped of its dielectric layer 4, has a circulation ratio of only about 17 db at its midfrequency, compared with a corresponding ratio of 35 to 40 db measured in a device according to the invention as described more particularly with reference to FIG. 2.

We claim:

1. In a system for the nonreciprocal transmission of microwave energy from an input end of a waveguide to an out-put end thereof, the combination with said waveguide of a source of a unidirectional magnetic field transverse to the direction of wave propagation from said input end to said output end and a gyrotropic body disposed in said waveguide in the region of said magnetic field for intercepting transmitted microwaves, said body comprising a solid ferrite core having an axis aligned with said field and a substantially uniform cross-section perpendicular to said axis, and a dielectric layer on said core encircling said axis, said layer having a dielectric constant ranging between substantially 2 to 10, the thickness of said layer ranging between substantially 10% and 30% of the largest dimension of said cross-section.

2. The combination defined in claim 1 wherein said layer is of uniform thickness all around said core.

3. The combination defined in claim 1 wherein said waveguide has n coplanar branches including a first branch forming said inlet and a second branch forming said outlet, 11 being greater than 2, said branches having a junction in the region of said magnetic field, said core having an n-sided polygonal profile with corners confronting the respective branches.

4. The combination defined in claim 3 wherein said cross-section is an equilateral triangle.

5. The combination defined in claim 1 wherein said body has a pair of nonconductive end plates projecting laterally beyond said core, said layer being integral with said end plates.

6. In a circulator comprising a waveguide structure with n coplanar sections converging at a central junction, n being greater than 2, the combination therewith of a source of unidirectional magnetic field transverse to the plane of said sections and a gyrotropic body disposed in said waveguide structure at said junction for intercepting microwaves traveling through any of said sections toward said junction; said body comprising a solid ferrite core having an axis aligned with said field and a substantially uniform cross-secton in the form of an n-sided polygon perpendicular to said axis, a dielectric layer on said core encircling said axis, the thickness of said layer being a minor fraction of the largest dimension of said cross-section, and a pair of nonconductive end plates with n-sided polygonal outline geometrically similar to but larger than said profile and projecting laterally on all sidesbeyond said layer, said end plates being integral with said layer and extending parallel to said plane within said structure.

7. The combination defined in claim 6 wherein said profile and said outline are concentric equilateral triangles.

References Cited UNITED STATES PATENTS 3,063,028 11/1962 Weiss 33324.2 3,104,361 9/1963 Leetmaa et al. 333-1.1

HERMAN KARL SAALBACH, Primary Examiner.

ELI LIEBERMAN, Examiner.

P. L. GENSLER, Assistant Examiner. 

1. IN A SYSTEM FOR THE NONRECIPROCAL TRANSMISSION OF MICROWAVE ENERGY FROM AN INPUT END OF A WAVEGUIDE TO AN OUTPUT END THEREOF, THE COMBINATION WITH SAID WAVEGUIDE OF A SOURCE OF A UNIDIRECTIONAL MAGNETIC FIELD TRANSVERSE TO THE DIRECTION OF WAVE PROPAGATION FROM SAID INPUT END TO SAID OUTPUT END AND A GYROTROPIC BODY DISPOSED IN SAID WAVEGUIDE IN THE REGION OF SAID MAGNETIC FIELD FOR INTERCEPTING TRANSMITTED MICROWAVES, SAID BODY COMPRISING A SOLID FERRITE CORE HAVING AN AXIS ALIGNED WITH SAID FIELD AND A SUBSTANTIALLY UNIFORM CROSS-SECTION PERPENDICULAR TO SAID AXIS, AND A DIELECTRIC LAYER ON SAID CORE ENCIRCLING SAID AXIS, SAID LAYER HAVING A DIELECTRIC CONSTANT RANGING BETWEEN SUBSTANTIALLY 2 TO 10, THE THICKNESS OF SAID LAYER RANGING BETWEEN SUBSTANTIALLY 10% AND 30% OF THE LARGEST DIMENSION OF SAID CROSS-SECTION. 